Design of electro-mechanical transducer elements



M. M. PALO DESIGN OF ELECTRO-MECHANICAL TRANSDUCER ELEMENTS Filed Nov.18, 1954 INVENTOR.

MAURICE u. PALO BY ATTOR EY DESIGN OF ELECTRO-MECHANICAL TRANSDUCERELEMENTS Maurice M. Palo, Conneaut, Ohio, assignor to The AstaticCorporation, Conneaut, Ohio, at corporation of Ohio Application November18, 1954, Serial No. 469,617

6 Claims. (Cl. 310-9.6)

The present invention relates generally to the art of convertingmechanical vibrations and movements 1nto electrical signals, and hasparticular reference to improvements in the design of piezoelectrictransducer elements of the type commonly employed in phonographapparatus, microphones, and the like.

As an overall object, the present invention seeks to providepiezo-electric transducer elements of novel and practical design, havingimproved physical and electrical characteristics rendering the elementsmore suitable for specific intended applications.

More specifically, it is an object of the present invention to providean improved transducer element which substantially approximates certaindesired ideal characteristics of a physical and/or electrical nature,while at the same time being capable of economical commercialmanufacture. In this respect, the invention contemplates the provisionof an improved transducer element which may be formed of extrudedceramic piezo-electric material, such as barium titanate, for example,and which is possessed of certain ideal characteristics heretoforeimpossible or difiicult of obtainment.

Another specific object of the invention resides in the provision of anovel and improved piezo-electric trans-' ducer element adapted uponbending or flexing thereof to generate electrical charges or signalswherein the charges or signals produced therein are uniform, orsubstantially so, over the entire extent of the active portion of thetransducer. It will be understood,.of course, that electrical charges orsignals generated by piezo-electrical elements are proportional to theseverity of the flexing or bending thereof. It follows that where thebending or flexing of the element is of changing severity over theactive extent of the element the charges produced in different areas orSections of the element will vary accordingly. Thus, it is one of theobjects of this invention to provide transducer elements of such designthat the stressing of the active piezo-electric portions thereof isuniform throughout, as is the electrical charge or signal producedthereby.

The above and other objects and advantages of the invention will becomeapparent upon full consideration of the following detailed specificationand accompanying drawing wherein are disclosed certain preferredembodiments of the invention.

In the drawing:

Figures 1-4 are enlarged cross section views of certain piezo-electricelements of heretofore known design;

Figure 5 is an enlarged cross section view of a piezoelectric element oftheoretical design, having certain ideal characteristics;

Figure 6 is an enlarged cross section view of a piezoelectric elementembodying the teachings of my invention and closely approximating thetheoretically ideal design of Figure 5;

Figure 7 is -a perspective view of a modified element of the type shownin Figure 6;

ate nt Figure '8 is a perspective view of a further modification of theelement shown in Figure 6;

Figures 9 and 10 are enlarged section views of transducer elementsincorporating further teachings of my invention; and

Figures 11 and 12 are perspective views of further modifications of thetransducer element of Figure 9.

Referring now to the drawing, and initially to Figures 14 thereof, thenumeral 10 designates an extruded cerramic piezo-electric member ofsubstantially circular tubular cross section. The member so representedis a common type of element, known in the art, and is provided with acontinuous inner electrode surface 11 and separate outer electrodesurface portions 12 and 13. The complete element is in the form of atube,.which, upon side-to-side bending movements is placed undercompression along one side and under tension along the opposite side,the arrangement being such that electrical charges of opposite polarityare built up along the opposite sides of the element, the charges beingdrawn oif through the electrodes 12 and 13 for amplification in any wallknown manner. It will be observed, of course, that in any side-to-sidebending movements about a vertical axis the extreme side portions of theelement will be most greatly flexed, while the upper and lower portions,being close to the axis of flexure, will be but slightly aifected.

In the element of Figure 2 the tubular ceramic body member is of a moreor less oval cross section, having flattened side portions, so thatduring side-to-side bending movements about a vertical axis there is amore uniform stressing of the side wall portions over any crosssectional portion of the element. Moreover, the structuralcharacteristics of the element of Figure 2 are more desirable forcertain purposes since, as will be readily observed, there will besubstantial compliance to bending in transverse directions, with littlecompliance to bending in vertical directions. As in the case oftheelement of Figure 1, a continuous inner electrode 111, and separateouter electrodes 112 and 113 are provided, the inner electrode beingemployed in initially polarizing the element while the outer electrodesare employed to pick up the charges or signals developed during flexingof the element.

In Figure 3, there is an essentially oval extruded ceramic body member210 having outer electrodes 212 and 213 and a plurality of corefilaments 211 in tubular openings 211 provided therefor. In thisinstance, the core filaments 211 take the place of the inner electrode.Otherwise, in its essential characteristics, the element of Figure 3 isequivalent to that of Figure 2.

In Figure 4 there is shown a transducer element having preferredoperating characteristics, but which is somewhat undesirable from thestandpoint of cost of manufacture. In this element a pair of flatceramic plates 310 and 310 are provided with outer electrode coatings312 and 313 and inner electrode coatings 311. The plates 310 and 310'are soldered or otherwise bonded to a center plate or vane 315 of brassor similar material. The Vane 315 constitutes a neutral plane aboutwhich the ceramic plates are bent to generate the desired electricalsignals.

In Figure 5 there is shown a theoretically ideal transducer assemblywherein two flat ceramic piezo-electric plates 410 and 410', having theproper electrode coatings 411, 412 and 413, are maintained in relativelywidely spaced relation by means of a theoretical infinitely compliantmaterial, indicated by broken lines 414. In this theoreticallyidealelement lateral or side-to-side bending forces will efficiently placethe spaced ceramic plates under tension or compression, depending uponthe direction of the bending force; and, as will be readily understood,

the tension and compression forces so produced Will be substantiallyuniform throughout the whole of the cross section of the plates 410 and410', as compared to the element of Figure 4, for example, where bendingforces will cause negligible tension or compression forces adjacent thevane 315, constituting the neutral bending p ane.

In accordance with the teachings of the present invention, asrepresented in the embodiment of Figure 6, I may realize the variousproduction and other advantages inherent in the designs of Figures 1-3,while realizing the various operating advantages of the designs ofFigures 4 and 5. Thus, in the cross sectional view of Figure 6 there isshown a transducer element which has a body portion of H-shaped section,which is preferably formed by extrusion processes or suitable ceramicpiezoelectric material. The vertically extending flanges 510 and 510' ofthe H-shaped section constitute the active operating portions of thetransducer element, and are equivalent to the spaced ceramic plates 410and 410' of the theoretically ideal transducer element of Figure 5.Suitable electrode coatings 512 and 513 are provided upon the outersurfaces of the flanges 510 and 510' in the conventional manner.

Connecting the flanges 510 and 510, and maintaining the same in spacedrelation, is a center web section 515 which is preferably of a minimumthickness consonant with the required physical strength of the elementfor the application intended. An inner electrode is of course requiredfor initially polarizing the element, and for this purpose separateconductive coatings 514 are provided about the upper and lower insidesurfaces of the H-shaped section, in the manner shown in Figure 6.

It will be observed that the transducer element of Figure 6 closelyapproximates the theoretically ideal section of Figure 5, being modifiedonly by the provision of a thin connecting web 515. The arrangement issuch that an eflicient push-pull stress relation is produced duringside-to-side bending movements of the transducer element so that mosteffective use is made of the active portions of the element.

In Figure 7 there is shown a modification of the element of Figure 6wherein further provisions have been made to insure even distribution ofbending stresses throughout the whole of the element. In manyapplications of piezo-electric elements the same are fixedly mounted atone end while being drivingly connected at the other end to a movablemember such as the record engaging stylus of a phonograph apparatus, forexample. The element is thus supported in the manner of a cantileverbeam, and when the free end is flexed bending stresses are set up withinthe beam or element which increase from the free to the fixed end of theelement. And, of course, since the magnitude of the electrical signalgenerated varies in accordance with the severity of the stress imposedupon the ceramic material the signal will vary over the unsupportedlength of the element. In such cases the full capacity of the element isnot realized, and in some instances the fidelity of reproduction may beless than could or should be realized from the element.

In the element of Figure 7 the upper end of the element, designated asB, would be the fixed end in a complete transducer assembly, while thelower end, desig nated as A, would be the free end, being connected bysuitable means to a phonograph stylus or other detecting device, notshown. As will be observed, the web member 515 is provided with a seriesof holes or apertures 516 -516 along its length, the size of theapertures de creasing from the free end A toward the fixed end B of theelement. In this manner the physical strength of the element isprogressively reduced toward the free end A; and when sideto-sidevibratory forces are applied to the end A the element will be flexedsubstantially uniformly along its entire length. It is thus possible torealize a maximum effective operating efliciency from the transducerelement.

In the embodiment of Figure 8 there is shown another embodiment of theelement of Figure 6 wherein means are afforded for utilizing the areasof the transducer element which are most greatly stressed duringoperation thereof. Thus, in the embodiment of Figure 8 the element is ofsubstantially uniform strength throughout its length, so that whenassembled as a cantilever beam, with the fixed end at B and the free endat A, stresses caused by side-to-side forces applied at the free end Awill cause the greatest stresses at and adjacent to the fixed end B.Accordingly, I propose to provide only the portion of the elementadjacent the fixed end B with conductive electrode coatings 512 and 513.As will be readily understood, this will render the element moresensitive or responsive to movements of the free end A, but willcorrespondingly reduce the electrical capacity of the overall unit.Therefore, the requirements of various particular applications willdetermine the most proper balance of the two factors.

Referring now to Figure 9, there is shown a preferred type of transducerelement having special contours so that when bending forces are appliedthereto a uniform stress is created in the active ceramic portionsthereof in the manner desired. The element of Figure 9 is fixedlymounted at its left hand end B, in any suitable manner,

and is connected at or near its free end A with a record engaging stylusor other device for detecting vibratory mechanical movements. The centerportion of the element 715 may be of brass or other conductive materialand is provided along each outer surface with a layer of ceramicpiezoelectric material, as indicated at 710 and 710'. At the outersurfaces of the ceramic layers 71%) and 710' are provided conductiveelectrode coatings 712 and 713, the conductive vane or center member 715constituting an inner electrode for polarization purposes in this case.

In accordance with the teachings of the invention the vane 715 and/orceramic layers 710 and 710' are so contoured with respect to the loadapplied thereto that upon application of the load the transducer elementwill bend uniformly throughout its length. In this manner the ceramiclayers are uniformly stressed throughout and most eflicient use is madeof the element. It will be understood, of course, that actuallongitudinal cross section of the element taken along the planeillustrated will vary according to the type and manner of application ofapplication of the side-to-side force, as well as the longitudinal crosssection taken along other planes. The basic principles of forming a beamhaving uniform strength in bending are well known in the structuralarts, for example, and in general these same principles are applied inthe present case in obtaining a beam-like transducer element having aprimary characteristic of uniform strength in bending throughout itsentire active length.

In Figures 11 and 12 are shown modifications of the element of Figure 9wherein the element is anchored at opposite ends, B and B, with thevibratory activating force being applied intermediate such fixed ends.The general structure of the elements of Figures 11 and 12 is similar tothe element of Figure 9, having reference again to the nature andapplication of forces thereto in order to arrive at a cross sectionwhich uniformly resists bending throughout its unsupported length.

In Figure 10 there is shown a further modification of the element ofFigure 9 wherein the center portion or vane 815 is formed with a portionof reduced cross section adjacent its fixed end B. The arrangement issuch that upon application of lateral forces at the outer or free end Aof the element the bending stresses are highly concentrated at theportion of reduced cross section. Electrode coatings 812 and 813 areapplied only at the area of reduced cross section, and accordingly theelement is highly responsive or sensitive to any movements of the outerend A. In addition, the area or reduced cross' section is so contoured,in accordance with the teachings set forth above, that the stressesproduced therein are substantially constant throughout the entire lengthof the area.

It should thus be apparent that I have fulfilled the objects initiallyset forth. The invention consists basically in the provision oftransducer elements having improved physical and electricalcharacteristics, and to this end I have provided a novel ceramicpiezoelectric element of H-shaped cross section having operatingcharacteristics closely approximating a theoretically ideal transducerelement, and having the further important advantage of being capable ofmanufacture by econornical extrusion processes. The element referred tois so arranged that a substantially uniform push-pull stressing of theactive elements results at any cross sectional layer thereof duringside-to-side bending movements of the element.

It should be particularly observed that since the piezoelectric elementsof the present invention are of the ceramic type, preferably of bariumtitanate, the lattice structure of the material is polycrystalline andas such can be and is polarized in desired orientations through outvarious desired sections of the physical body. Thus, in the body ofFigure 6, for example, it is possible to orient the polerization in thetwo slab sections 510 and 510' in such a manner that the generativeelfect across the thickness of these slabs upon bending of the elementas herein discussed is in series and thus additive when the electricaloutput of the element is taken from the two outer electrodes 512 and513. Such polarization of piezo-electric elements comprising spacedslabs of polycrystalline material is in accordance with standardpractice and is well known to those skilled in the art as is evidencedby the prior art patent to Howatt, No. 2,640,165.

In addition, the invention teaches a novel arrangement for proportioninga transducer element throughout its length so that the bending stressestherein are uniform under the load applied. In this manner the electrical charges produced are uniform throughout the length of theelement, and a maximum operating capacity may be realized therefrom. Incombination with this latter feature, or in some cases independentlythereof, I may provide a conductive electrode coating over only aportion of a complete transducer element, that portion being at andadjacent the area at which the greatest stress occurs during bending ofthe element. By this arrangement the transducer is rendered accuratelyresponsive to applied bending forces.

It will be understood, however, that the embodiments herein shown andspecifically described are intended to be illustrative only, andreference should therefore be made to the following appended claims indetermining the full scope of the invention.

I claim:

1. A piezo-electric transducer element comprising spaced layers ofpolarized polycrystalline piezo-electric material disposed insymmetrical relation to a principal longitudinal neutral plane, meansinterconnecting said spaced layers, said element being adapted to befixedly mounted at one end portion and flexed at its other end portion,and electrode means covering only portions of said element which uponbending of the electrode are subtsantially uniformly stressed.

2. An element according to claim 1 wherein said electrode means coveronly a portion of said element adjacent the fixed end thereof.

3. A piezoelectric transducer element comprising spaced parallel layersof polarized polycrystalline piezo electric material disposed insymmetrical relation to a principal longitudinal neutral plane, a thinweb-like member disposed transversely to said layers and interconnectingthe same at their inner principal surfaces, each of said layers havingelectrode means on their outer principal surfaces, said element beingadapted to be fixedly connected at one end portion and flexed at itsother end portion, and said web-like member being structurally weakenedprogressively toward the free end of said element.

4. A piezo-electric transducer element comprising spaced parallel layersof polarized polycrystalline piezoelectric material disposed insymmetrical relation to a principal longitudinal neutral plane, a thinweb-like member disposed transversely to said layers and interconnectingthe same at their inner principal surfaces, each of said layers havingelectrode means on their outer principal surfaces, said element beingadapted to be fixedly connected at one end portion and flexed at itsother end portion, and said web-like member having a plurality ofapertures therein of-progressively decreasing size in a direction fromthe free end of said element to the point at which said element ismounted.

5. A piezo-electric transducer element comprising spaced parallel layersof polarized polycrystalline piezoeletcric material disposed insymmetrical relation to a principal longitudinal neutral plane, a thinweb-like member disposed transversely of said layers and interconnectingthe same at their inner principal surfaces, each of said layers havingelectrode means of their outer principal surfaces, said element beingadapted to be fixedly connected at one end portion and flexed at itsother end portion, and said electrode means covering only a portion ofsaid element adjacent the fixed end thereof.

6. A piezo-electric transducer element comprising spaced layers ofpolycrystalline piezo-electric material disposed in spaced relation to aprincipal longitudinal neutral plane, a thin web-like member disposedtransversely to said layers and interconnecting the same at their innerprincipal surfaces, each of said layers having electrodes on its innerand outer principal surfaces whereby said spaced layers may be initiallypolarized so that upon bending of said element in a directiontransversely of said layers and said neutral plane the generative eifectacross said electrodes on said outer principal surfaces of said layersis in series and additive.

References Cited in the file of this patent UNITED STATES PATENTS Re.20,213 Sawyer Dec. 22, 1936 1,925,577 Ttillyer Sept. 5, 1933 2,640,165Howatt May 26, 1953 FOREIGN PATENTS 847,744 France July 10, 1939

